Splenomegaly is commonly found in mice bearing tumor grafts. In order to study this relationship further, attempts were made to transfer the splenomegaly to nontumor bearing animals. Splenic extract from two different strains of tumor bearing mice were injected into normal mice of the same strains every other day for varying periods of time. The injected normal mice developed splenomegaly. The induced splenomegaly was transferrable through six successive “generations” of animals. Mice were made hypersplenic and then inoculated with tumor grafts. The splenomegaly had no effect on the growth of the tumor. It was found that the active fraction in the tumor bearing splenic extract was completely destroyed by heating to 65 C for 30 min and dialysis across cellophane against cold water resulted in nearly complete loss of activity. Controls were injected with nromal splenic extract or sham injected with saline. Strain specificities were also tested. Splenic indices were calculated for all mice.